Intermodulation products or intermodulation distortions (IMD) occur in nonlinear systems and therefore in real technical systems when two or more signals of different frequencies are processed. It can thus occur that during mixing on a nonlinear circuit element, products of the two signals are generated. In the case of two signals of the formU1(t)=A1 cos(ω1t)andU2(t)=A2 cos(ω2t)a plurality of frequency components mω1+/−nω2 result with m, n=0, 1, 2, . . . , wherein the sum m+n specifies the order. If a resulting frequency component is a multiple of a fundamental tone ω1 or ω2, this is referred to as a harmonic. If it is the total of multiples of the two fundamental tones, it is referred to as an intermodulation product.
In multiplexers and especially in duplexers, those intermodulation products are problematic which arise at an antenna input and are located in the Rx band or in the vicinity of the Rx band. They “block” the Rx signal path, since they cannot simply be filtered out by filtering measures. Otherwise, the Rx useful frequency would also be destroyed. Such undesired intermodulation products can arise in particular in duplexers due to the multiplication of Tx signals with a blocker signal received externally via the antenna. The Rx passband associated with a Tx signal is relatively close, usually above the Tx passband, in the case of a duplexer. Therefore, harmonics of the Tx signal do not fall, but intermodulation products of the Tx signal and an externally received signal do fall in its own Rx passband.
In comparison to harmonics which can be removed by high-pass filters or local poles in the stopband of a transmit filter, a way other than filtering out must therefore be selected.
Providing BAW resonators, which are connected in antiparallel to one another, to reduce nonlinear effects is known from DE 10 2005 028 927 A1.
This has the disadvantage that space on the surface of a costly chip must be provided for the additional resonators.